algaesingular algamembers of a group of predominantly aquatic, photosynthetic organisms of the kingdom Protista. They range in size from the tiny flagellate Micromonas that is 1 micrometre (0.00004 inch) in diameter to giant kelp kelps that reach 60 metres (200 feet) in length. Algae provide much of the Earth’s oxygen, they are the food base for almost all aquatic life, they are an original a source of petroleum productscrude oil, and they provide foods and pharmaceutical and industrial products for humans. The algae have many types of life cycles, from simple to complex. Their photosynthetic pigments are more varied than those of plants, and their cells have features not found among plants and animals. Some groups of algae are ancient, while whereas other groups appear to have evolved more recently. The taxonomy of algae is changing rapidly because so much subject to rapid change because new information is constantly being discovered. The study of algae is termed phycology, and one who studies algae is known as a phycologist.

In this article the algae are defined as eukaryotic (nucleus-bearing) organisms that photosynthesize but lack the specialized reproductive structures of plants, which always have multicellular reproductive structures that contain fertile gamete-producing cells surrounded by sterile cells. Algae lack true roots, stems, and leaves—features they share with the plant division Bryophyta (e.g., mosses and liverworts).

The algae as treated in this article do not include the prokaryotic (nucleus-lacking) blue-green algae (cyanobacteria) and prochlorophyte algaeor prochlorophytes. Beginning in the 1970s, some scientists suggested that the study of the prokaryotic algae should be incorporated into the study of bacteria because of certain shared cellular features. Other However, other scientists consider the oxygen-producing photosynthetic capability of blue-green and prochlorophyte algae to be as significant as cell structure. Therefore, and they these organisms continue to classify them as algae.

In this article the algae are defined as eukaryotic (nucleus-bearing) organisms that photosynthesize but lack the specialized reproductive structures of plants. Plants always have multicellular reproductive structures where the fertile, gamete-producing cells are surrounded by sterile cells; this never occurs in algae. Algae lack true roots, stems, and leaves, but they share this feature with the plant division Bryophyta (e.g., mosses and liverworts).

be classified as algae.

Beginning in the 1830s, algae were classified into major groups based on colour (e.The g., red, brown, and greenseaweeds are well known to those who have walked along a rocky seashore). The colours are a reflection of the chloroplast pigment molecules—i.e., the different chloroplast pigments, such as chlorophylls, carotenoids, and phycobiliproteins. Many more than three groups of pigments are now recognized, and each class of algae shares a common set of pigment types that is distinct from those of all other groups.

The algae are not closely related in an evolutionary sense. Specific groups of algae share enough features with protozoa and fungi thatit is difficult to distinguish them from certain protozoa or fungi without using , without the presence of chloroplasts and photosynthesis as delimiting features, make them difficult to distinguish from certain protozoa and fungi. Thus, some algae appear to have a closer evolutionary relationship with the protozoa or fungi than they do with other algae, andthe converse is also true—some , conversely, some protozoa or fungi are more closely related to algae than to other protozoa or fungi.In fact, if the algae are united into one evolutionary group with a common ancestor, then that evolutionary group will include animals, fungi, plants, and protozoa as well.

Knowledge and use of algae are perhaps as old as mankind (Homo sapiens), and possibly earlier species such as H. erectus also knew of, and used, algae. humankind. Seaweeds are still eaten by some coastal societiespeoples, and algae are considered acceptable foods in many restaurants. Anyone who has slipped on a “slimy” rock (covered with diatoms) while crossing a stream has had firsthand experience with algae. Others know algae as green Many slimy rocks are covered with algae such as diatoms or cyanophytes, and algae are the cause of green or golden sheens on pools and ponds. The algae Algae are the base of the food chain for all marine organisms since most plants do not few other kinds of plants live in the oceans.Because the oceans occupy about 71 percent of the Earth’s surface area, the role of algae in supporting aquatic life is essential.

This article discusses the algae in general termsterms of their morphology, ecology, and evolutionary features. For a discussion of the related protists, seethe articles protozoan and protist. For a more complete discussion of photosynthesis, seethe articles photosynthesis and plant.

General Physical and ecological features of algae

Size range and diversity of structure

Algae span The size range of the algae spans seven orders of magnitudein size. Many algae consist of only one cell, others have two or more cells, and the largest have millions of cells. In large, macroscopic algae, groups of cells are specialized for specific functions, such as anchorage, transport, photosynthesis, and reproduction. Specialization involving thousands of cells is sometimes interpreted as indicates a measure of complexity and evolutionary advancement.An alga that can accomplish the same functions using only one cell, however, should not be interpreted as a “simple” alga.

The algae are artificially The algae can be divided into several types based on the morphology of their vegetative, or growing, state(Figures 1, 2, and 3). Filamentous forms have cells arranged in chains and an overall appearance like strings or filamentsof beads. Some filaments (like Spirogyra, Figure 1e.g., Spirogyra) are unbranched, while whereas others (e.g., like Stigeoclonium, ) are branched and appear bushlike. In many red algae—for examplealgae (e.g., Palmaria—numerous ), numerous adjacent filaments lie together and joined laterally create the gross morphological form of the alga. Parenchymatous (tissuelike) forms, such as the giant kelp Macrocystis, can become be very large. Other , measuring many metres in length. Coenocytic forms of algae grow to large sizes without forming distinct cells(i.e., the protoplasm flows together as a large living mass); the green seaweed Codium, commonly known as . Coenocytic algae are essentially unicellular, multinucleated algae in which the protoplasm (cytoplasmic and nuclear content of a cell) is not subdivided by cell walls. The green seaweed Codium, which has been called dead-man’s-fingers, is an example of this. Some algae have flagella and swim through the water. These flagellates (Figure 1) range from single cells, such as in Ochromonas, to colonial organisms with thousands of cells, such as Volvox. Coccoid and capsoid algae (Figure 1) are unicellular or multicellular. Coccoid organisms, such as Scenedesmus, normally have an exact number of cells per colony, and that number is produced by a series of rapid cell divisions when the organism is first formed; once the exact cell number is obtained, the organism grows in size but not in cell number. Capsoid organisms, such as Chrysocapsa, have variable numbers of cells; the . These cells are found in clusters that increase gradually in cell number increases gradually, and the cells are embedded in a copious transparent gel.

Distribution and abundance

Algae are almost ubiquitous , although they are throughout the world, being most common in aquatic habitats. Algae are often They can be categorized ecologically by their habitathabitats. Planktonic microscopic algae are grow suspended in the water, while whereas neustonic algae grow on the water surface. Cryophilic algae grow occur in snow and ice, while ; thermophilic algae live in hot springs. Epidaphic ; edaphic algae live on soil, and endedaphic algae live in soil. Algae can live attached to other organisms: or in soil; epizoic algae grow on animals, such as turtles , polar bears, and tree sloths; epiphytic algae grow on fungi, land plants, or other algae. Algae can be classified even more specifically. For example, ; corticolous algae grow on the bark of trees. Epilithic ; epilithic algae live on rocks, ; endolithic algae live in porous rocks, ; and chasmolithic algae grow in rock fissures. Some algae live inside other organisms, and in a general sense these are called endosymbionts. Specifically, endozoic endosymbionts live in protozoa or other, larger animals, while whereas endophytic endosymbionts live in fungi, plants, or other algae.Algae also live in the atmosphere, moving with air currents and receiving moisture and nutrients from clouds.

Algal abundance and diversity varies vary from one environment to the next, just as land plant abundance and diversity varies vary from tropical rain forests to deserts. Terrestrial vegetation (plants and algae) is influenced most by precipitation and temperature, while whereas aquatic vegetation (primarily algae) is influenced most by light and nutrients. When nutrients are abundant, as occurs in some polluted waters, algal cell numbers can become great enough to produce obvious patches of algae . These patches are called “blooms” or “red tides.” The number of organisms necessary for a bloom varies with the size of the organism and the concentration of cells. Blooms are ,” usually linked to favourable growing conditions, including an abundance of nutrients, but blooms do not occur every time favourable conditions exist, and therefore it is difficult to explain or predict their occurrence.

Ecological and commercial importance

Algae use photosynthesis to form organic food molecules from carbon dioxide and water . Like through the process of photosynthesis, in which they capture energy from sunlight. Similar to land plants, algae are located at the base of the food webchain, and the existence of nonphotosynthetic organisms depend upon this photosynthetic food base for their existenceis dependent upon the presence of photosynthetic organisms. Nearly three-fourths of Earth is covered by water, and since the so-called higher plants are virtually absent from the major water sources (e.g., the oceans. Marine life— including ), the existence of nearly all marine life—including whales, seals, fishes, turtles, shrimps, lobsters, clams, octopuses, starfish, and worms—ultimately depends upon algaefor its existence. In addition to making organic molecules, algae produce oxygen as a by-product of photosynthesis. Algae produce an estimated 30 to 50 percent of the net global oxygen . This oxygen is available to humans and other terrestrial animals for respiration, and it is consumed when coal, wood, or oil is burned.Although terrestrial ecosystems also produce large amounts of oxygen, the organisms living in these ecosystems consume it relatively rapidly on a geologic time scale

, so that the net oxygen production by rain forests over time is low.Crude oil and natural gas are the remnants of the photosynthetic products of ancient algae, which were subsequently modified by bacteria. The North Sea oil deposits were are believed to have been formed from coccolithophore algae (class Prymnesiophyceae), and the Colorado oil shales were produced by an alga similar to Botryococcus. Today, Botryococcus(a green alga). Today, Botryococcusproduces blooms in Lake Baikal , in Russia, and produces large amounts of oil. The oil is so abundant on where it releases so much oil onto the surface of the lake that it is can be collected with a special skimming apparatus and used as a source of fuel. Several companies have grown oil-producing algae in high-salinity ponds and have extracted the oil as an a potential alternative to fossil fuels.

Algae, as processed and unprocessed food, have a an annual commercial value of several billion dollarsannually. Approximately 500 species are eaten by humans, and some 160 are commercially important. In addition to the use of algal extracts in prepared foods (see below), algae are eaten directly in many parts of the world. Algae are a significant food item in the Algal extracts are commonly used in preparing foods and other products, and the direct consumption of algae has existed for centuries in the diets of East Asian and Pacific Island societies, and unprocessed algae are eaten by South Americans, North Americans, and northern Europeans. Hawaiians have the most diverse diet of algae. In the 1800s at least 75 species were eaten by Hawaiians, and in the 1980s more than 50 species of algae were still being consumed. Many species of algae, including Porphyra umbilicalis (nori, or laver) and Palmaria palmata (dulse), are eaten by humans. The red alga Porphyra is the most important commercial food alga. In Japan alone approximately 100,000 hectares (247,000 acres) of shallow bays and seas are farmed. Porphyra has two major stages in its life cycle, : the Conchocelis stage and the Porphyra stage. The Conchocelisstage is a small, shell-boring stage that is can be artificially propagated by seeding on oyster shells that are tied to ropes or nets and set out in special marine beds for further development. The mature Conchocelis produces conchospores that germinate and grow into the large Porphyra stage. Either the oyster shells are tied to ropes or nets, or the conchospores are seeded directly onto the ropes or nets. The large, mature Porphyra blades are blades of Porphyra plants, which in due course are removed from the netsand , washed, sometimes chopped, and pressed into sheets to dry.

Palmaria palmata, another red alga, is eaten primarily in the North Atlantic region. Known as dulse in Canada and the United States, as duileasg (dulisk) in Scotland, as duileasc (dillisk) in Ireland, and as söl in Iceland, it is harvested by hand from intertidal rocks during low tide. Laminaria species (Species of Laminaria, Undaria, and Hizikia (a type of brown algae) are also harvested from wild beds along rocky shores, particularly in Japan. Laminaria is , Korea, and China, where they may be eaten with meat or fish and in soups. The green algae Monostroma and Ulva look somewhat like leaves of lettuce leaves (their common name is sea lettuce) , and they are eaten as salads or in soups, relishes, and meat or fish dishes.

The microscopic, freshwater green alga Chlorella is cultivated as a food supplement and is eaten in Taiwan, Japan, Malaysia, and the Philippines. It has a high protein content (53–65 53 to 65 percent) and has even been considered as a possible food source during extended space travel.

The cell walls of many seaweeds contain phycocolloids that have received increasing use in prepared foods. The term phycocolloid means “algal colloid” and is based upon the colloidal substances that are extracted from cell walls. The usefulness of these compounds is based on the colloidal property of alternating between sol (fluid) and gel (solid) states. The (algal colloids) that can be extracted by hot water. The three major phycocolloids are alginates, agars, and carrageenans. Alginates are extracted primarily from brown seaweeds, and agar and carrageenan are extracted from red seaweeds. These phycocolloids are cell-wall polysaccharides that are formed by the polymerization polymers of chemically modified galactose sugar molecules(, such as galactose in agars and carrageenans) , or of organic acids, such as mannuronic acid and glucuronic acid (in alginates). Phycocolloids are . Most phycocolloids can be safely consumed by humans and other animals, and many are therefore used in a wide variety of prepared foods, such as “ready-mix” cakes, “instant” puddings and pie fillings, and artificial dairy toppings.

Alginates, or alginic acids, are commercially extracted from brown seaweeds, especially the kelp such as Macrocystis, Laminaria, and Ascophyllum. Alginates , are used in ice creams to limit ice crystal formation , thereby (producing a smooth texture, and are also used ), in syrups as emulsifiers and thickenersin syrups , and as fillers in candy bars and salad dressings as fillers.

Agarsare , extracted primarily from species of the red alga algae, such as Gelidium, but they are also obtained from other red algae, especially Gracilaria, Pterocladia, Acanthopeltis, and Ahnfeltia. Agars , are used in instant pie fillings, canned meats or fish, and bakery icings . Agar is also used as a clarifying agent in and for clarifying beer and wine.

Carrageenans , from the Irish word “carraigin” (meaning Irish moss), are extracted from various red algae: , including Eucheuma in the Philippines, Chondruscrispus(also called Irish moss) in the United States and the Canadian Maritime Provinces, and Iridaea in Chile. Carrageenans are widely used in food products, and it It is estimated that the average human consumption of carrageenans in the United States is 250 milligrams mg (0.01 ounce) a day . Carrageenans in the United States, where they are used as for thickening and stabilizing agents in dairy products, imitation creams, puddings, syrups, and canned pet foods.

The diatoms have In addition to their important role as food products, phycocolloids have industrial uses. They are relatively inert and are used as creams and gels in medical drugs and insecticides. Agar is used extensively in laboratory research as a substrate for growing bacteria, fungi, and algae in pure cultures and as a substrate for eukaryotic cell culture and tissue culture. Carrageenans are used in the manufacture of shampoos, cosmetics, and medicines.

The diatoms (class Bacillariophyceae) played an important role in industrial development during the 20th century. The frustules, or cell walls, of diatoms (class Bacillariophyceae) are made of opaline silica and contain many fine pores. Occasionally, large Large quantities of frustules are deposited in some ocean and lake sediments, and the their fossilized remains are called diatomite. Diatomite contains approximately 3,000 diatom frustules per cubic millimetre (50 million diatom frustules per cubic inch). When geologic uplifting brings these deposits of diatomite above sea level, the diatomite is easily mined. Diatomite contains approximately 50 million diatom frustules per cubic inch. The deposit at A deposit located in Lompoc, Calif., U.S., for example, covers 13 square kilometres (5 square miles) and is up to 425 metres (1,400 feet) deep.

Diatomite is relatively inert . It and has a high absorptive capacity, large surface area, and low bulk density, and relatively low abrasion. It consists of approximately 90 percent silica, and the remainder consists of compounds such as aluminum and iron oxides. The fine pores in the diatom frustules make it diatomite an excellent filtering material for beverages (e.g., fruit juices, soft drinks, beer, and wine), chemicals (e.g., sodium hydroxide, sulfuric acid, and gold salts), industrial oils (e.g., those used as lubricants or in rolling mills or for cutting), cooking oils (e.g., vegetable and animal), sugars (e.g., cane, beet, and corn), water supplies (e.g., municipal, swimming pool, waste, and boiler), varnishes, lacquers, jet fuels, and antibiotics, to name a few. The as well as many other products. Its relatively low abrasive properties make it suitable for use in toothpaste, “nonabrasive” sink cleansers, polishes (for silver , automobileand automobiles), and buffing compounds.

Diatomite is also widely used as a filler and extender in paint, paper, and rubber, and plastic products; the . The gloss and sheen of “flat” paints can be controlled using by the use of various additions of diatomite. During the manufacture of plastic bags, diatomite is can be added to the newly formed sheets to act as an antiblocking agent so that the plastic (polyethylene) can be rolled while it is still hot. Because diatomite it can absorb approximately 2.5 times its weight in water, it also makes an excellent anticaking carrier for powders used to dust roses or for cleansers used to clean rugs. Diatomite is also used in making welding rods, battery boxes, concrete, explosives, and animal foods.

Chalk is another fossilized deposit of protistan remains of protists. It consists in part of calcium carbonate scales, or coccoliths, from the coccolithophore members of the class Prymnesiophyceae. Chalk deposits, such as the White Cliffs of Dover in England, white cliffs in Dover, Kent, Eng., contain large amounts of coccoliths, as well as the shells of foraminiferan protozoa. Coccoliths can be observed in fragments of ordinary blackboard chalk examined under a light microscope.

By the end of the 18th century, kelp kelps (class Phaeophyceae) was were harvested and burned as a means of producing soda commerciallyto produce soda. When mineral deposits containing soda were discovered in Salzburg, Austria, and elsewhere, the use of kelp ash declined. Kelp was Kelps were again harvested in abundance during the 19th century when salts and iodine were extracted for commercial use. Once again, however, newly discovered mineral deposits, this time of , although the discovery of cooking salt and iodides , led to a demise of the kelp industry. During the 20th century the kelp industry again flourished. During World War I the United States used kelp as a source of potash and acetone.Seaweeds have been seaweeds to produce potash, a plant fertilizer, and acetone, a necessary component in the manufacture of smokeless gunpowder.

For many centuries, seaweeds around the world have been widely used as agricultural fertilizersfor centuries in many parts of the world. Coastal farmers cut seaweeds that were collect seaweeds by cutting them from seaweed beds growing in the ocean or by gathering them from masses washed up on shores after storms. The seaweeds are then spread over the soil. Kelp is now used to extract macronutrients and micronutrients for specialized plant fertilizers and animal feed supplements. Dried kelp is Dried seaweed, although almost 50 percent mineral matter; Ascophyllum nodosum, for example, contains 55 trace elementscontains a large amount of nitrogenous organic matter. Commercial extracts used of seaweed sold as plant fertilizers contain a mixture of macronutrients, micronutrients, and trace elements that provide promote robust plant growth.

The metal and other trace elements are chelated (bound) to organic sugars so that the concentration of free metals, which can be toxic to plants and animals, is very low. The equilibrium between chelated and free forms provides a gradual release of nutrients from the extract (fertilizer) as plants use the free forms.

Phycocolloids have industrial uses in addition to their important roles in food products. Because they are relatively inert and have good gelling properties, they are used as creams and gels for carrying minute amounts of active additives, as in medical drugs or insecticides. Agar is used extensively as a bacteriologic culturing substrate in medical and research facilities and is also used as a substrate for eukaryotic cell and tissue culture, including the culture of algae themselves. Carrageenans are used in the manufacture of shampoos, cosmetics, and medicines.

The green unicellular flagellate Dunaliella, which turns red when physiologically stressed, is cultivated in saline ponds . The culture conditions are manipulated so that carotene or glycerol are for the production of carotene and glycerol. These compounds can be produced in large amounts . These compounds are and extracted and sold commercially.

Toxicity

Algae Some algae can be harmful to humans. Some algae A few species produce toxins that become may be concentrated in shellfish and finfish. Although they have little effect on shellfish and finfish, the toxins accumulate in the seafood flesh, making it , which are thereby rendered unsafe or poisonous for human consumption. The dinoflagellates (class Dinophyceae) are the most notorious producers of toxins. Paralytic shellfish poisoning is caused by saxitoxin or any of at least 12 related compounds. Saxitoxin is probably the most toxic compound knownand ; it is 100,000 times more toxic than cocaine. Saxitoxin and saxitoxin-like compounds are nerve toxins that interfere with neuromuscular junctionsfunction. Alexandrium tamarense and Gymnodinium catenatum are the two species most often associated with paralytic shellfish poisoning. Diarrheic shellfish poisoning is caused by okadaic acids that are produced by several kinds of algae, especially species of Dinophysis. Neurotoxic shellfish poisoningis , caused by toxins produced in Gymnodinium breve, a red tide organism. This alga an organism associated with red tides, is notorious for fish kills and shellfish poisoning along the coast of Florida , U.S. in the United States. When the red tide blooms are blown to shore, wind-sprayed toxic cells can cause health problems for humans and other animals that breathe the air.

Ciguatera is a disease of humans caused by consumption of tropical fish infected with the algae that have fed on the alga Gambierdiscus or Ostreopsis. Unlike many other dinoflagellate toxins, ciguatoxin and maitotoxin are concentrated in fish finfish rather than shellfish. Levels as low as one part per billion in fish are can be sufficient to cause human intoxicationsintoxication.

Not all shellfish poisons are produced by dinoflagellates. Amnesiac Amnesic shellfish poisoning is caused by domoic acid, which is produced by diatoms (class Bacillariophyceae), such as Nitzschia pungens f. multiseries, and Nitzschia pseudodelicatissima. Symptoms of this poisoning in humans progress from abdominal cramps to vomiting to memory loss to disorientation and finally to death.

Several algae produce toxins that are lethal to fish. Prymnesium parvum (class Prymnesiophyceae) has caused massive die-offs in ponds where fish are cultured, and Chrysochromulina polylepis (class Prymnesiophyceae) has caused major fish kills along the coasts of the Scandinavian countries. Other algae, such as Heterosigma (class Raphidophyceae) and Dictyocha (class Dictyochophyceae), are suspected fish killers as well.

Algae can cause human diseases by directly attacking human tissues, although the frequency is rare. Protothecosisis , caused by a the chloroplast-lacking green alga, Prototheca. The alga infects , can result in waterlogged skin lesions, grows subcutaneously, and can in which the pathogen grows. Prototheca organisms may eventually spread to the lymph glands from these subcutaneous lesions. Prototheca is also believed to cause be responsible for ulcerative dermatitis in the Australian platypus. Similar Very rarely, similar infections , in humans and cattle , are can be caused by chloroplast-bearing species of Chlorellaof the class Chlorophyceae.

Some seaweeds contain high concentrations of arsenic and when eaten may cause arsenic poisoning. For example, the brown alga Hizikiahas contains sufficient arsenic that it was to be used as a rat poisonin Asian countries.

Diatoms have been used in forensic medicine. Where In cases in which death by drowning is suspected, lung tissue is examined. The and blood vessels are examined; the presence of silica siliceous diatom cell walls can verify walls, transported in the bloodstream of the dying persons, is evidence for death by drowning; in mysterious cases, the . Certain diatom species can even be used to pinpoint the exact location of death because the species insofar as they are characteristic for a given lake, bog, or bay. Diatomite used in the manufacture of car polishes, paints, and matches, for example, is used in solving crimes as wellbody of water.

Form and function of algae

The algal cell

Eukaryotic algal cells contain three types of double-membrane-bound organelles: the nucleus, the chloroplast, and the mitochondrion. In most algal cells there is only a single nucleus, although some cells are multinucleate. In addition, some algae are siphonaceous, with many nuclei not separated by cell walls. The nucleus contains most of the genetic material, or deoxyribonucleic acid (DNA), of the cell. In most algae, and the molecules of DNA (deoxyribonucleic acid) molecules exist as linear strands . The DNA is that are condensed into obvious chromosomes only at the time of nuclear division (mitosis)in most algae; however, the nuclear DNA of the classes there are two classes of algae, Dinophyceae and Euglenophyceae, in which the nuclear DNA is always condensed . The into chromosomes. In all algae, the two membranes surrounding that surround the nucleus are referred to as the nuclear envelope, which . The nuclear envelope typically has specialized nuclear pores that regulate the movement of molecules into and out of the nucleus.

The chloroplast is the site Chloroplasts are the sites of photosynthesis, the complex set of biochemical reactions that convert light energy, use the energy of light to convert carbon dioxide , and water into sugars. The Each chloroplast contains flattened, membranous sacs, called thylakoids, that contain the photosynthetic light-harvesting pigments, the chlorophylls, carotenoids, or and phycobiliproteins (see belowPhotosynthesis).

The mitochondrion is mitochondria are the site sites where food molecules are broken down and carbon dioxide, water, and chemical bond energy are released, a process called cellular respiration (see belowCellular respiration). Photosynthesis and respiration are approximately opposite processes, the former building sugar molecules and the latter breaking them down. The inner membrane of the mitochondrion is infolded to a great extent, and this provides the surface area necessary for respiration. The infoldings, called cristae, have three morphologies: (1) flattened or sheetlike, (2) fingerlike or tubular, and (3) paddlelike. Plants The mitochondria of land plants and animals, by comparison, generally have only flattened cristae.

Chloroplasts and mitochondria also have their own DNA. This However, this DNA is not like nuclear DNA , however, because in that it is circular (or, more correctly, in endless loops) rather than linear , and therefore it resembles the DNA of prokaryotes. The similarity of chloroplastic and mitochondrial DNA to prokaryotic DNA has led many scientists to accept the hypothesis of endosymbiosis, which states that these organelles resulted from a developed as a result of a long and successful symbiosis symbiotic association of prokaryote cells inside eukaryote host cells.These symbioses have been used in defining the endosymbiosis hypothesis, which states that eukaryotic cells are formed in part by incorporating prokaryotes as specialized organelles.

Algae Algal cells also have several single-membrane-bound organelles, including the endoplasmic reticulum, Golgi apparatus, lysosomelysosomes, peroxisomeperoxisomes, vacuole, contractile vacuoleor noncontractile vacuoles, and, in some, ejectile organelles. The endoplasmic recticulum reticulum is a complex membranous system that forms intracellular compartments, acts as a transport system within the cell, and serves as a site for synthesizing fats, oils, and proteins. The Golgi apparatusis , a series of flattened, membranous sacs that are stacked like pancakes. The Golgi apparatus arranged in a stack, performs four distinct functions: it sorts many molecules synthesized elsewhere in the cell; it produces carbohydrateslike , such as cellulose or sugars, and sometimes it attaches the sugars to other molecules; it packages molecules in small vesicles; and it marks the vesicles so that they are routed to the proper destination. The lysosome is a specialized vacuole that contains digestive enzymes used to that break down old organelles, cells or cellular components during certain developmental stages, and particulate matter that is ingested by in species that can engulf food. Peroxisomes specialize in metabolically breaking down certain organic molecules and in destroying dangerous peroxide compounds, such as hydrogen peroxide, that are may be produced during some biochemical reactions. Vacuoles are membranous sacs that store many different substances, depending upon on the organism and its metabolic state. The contractile vacuole is Contractile vacuoles are specialized organelles that regulate the water content of cells and are therefore not involved in the long-term storage ; rather, it is a highly specialized organelle that regulates the water content of cellsof substances. When too much water enters the cells, the contractile vacuole “squirts” contractile vacuoles serve to “squirt” it out. Some algae have special ejectile organelles that apparently act as protective structures. The Dinophyceae has trichocysts—harpoonlike structures that lie harpoonlike trichocysts beneath the cell surface and that can explode from the a disturbed or irritated cell. Ejectosomes, of analogous structure, Trichocysts may serve to attach prey to algae cells before the prey is consumed. Ejectosomes are structures that are analogous to ejectile organelles and are found in the class Cryptophyceae. Several classes of algae in the division Chromophyta have mucous organelles that secrete slime. Gonyostomum semen, a freshwater member of the class Raphidophyceae, has numerous mucocysts, and when it is which, when such cells are collected in a plankton netthe mucocysts discharge, giving , discharge and render the net and sample a mucous consistencyits contents somewhat gummy.

The nonmembrane-bound organelles of algae include the ribosomes, pyrenoids, microtubules, and microfilaments. The ribosome serves as the “workbench” during protein synthesis. It provides the site where genetic information, as messenger RNA, is translated into proteins. The ribosome carefully interprets Ribosomes are the sites of protein synthesis, where genetic information in the form of messenger ribonucleic acid (mRNA) is translated into protein. The ribosomes accurately interpret the genetic code of the DNA so that the each protein is made exactly to the genetic specifications. The pyrenoid, a dense structure that occurs within inside or beside chloroplasts of certain algae, has a concentration consists largely of ribulose biphosphate carboxylase, one of the enzyme enzymes necessary in photosynthesis for carbon fixation and thus sugar formation(see below Photosynthesis). Starch, the a storage form of sugarglucose, is often found around pyrenoids. The microtubules are Microtubules, tubelike structures formed from tubulin proteins. Some microtubules , are almost always present in the cell, but others appear suddenly when needed and then disassemble after usemost cells. In many algae, microtubules appear and disappear as needed. Microtubules provide a rigid structure, or cytoskeleton, in the cell that helps determine and maintain the shape of the cell, and especially in species without cell wallsthe cytoskeleton maintains the cell shape. Microtubules also provide a sort of “rail” system along which vesicles are transported. The spindle apparatus, which separates the chromosomes when the nucleus dividesduring nuclear division, consists of microtubules. Finally, certain kinds of microtubules also form the basic structure, or axoneme, of the a flagellum, and they are a major component of the flagellar root system that anchors the a flagellum in within the cell. Microfilaments are formed by the polymerization of proteins such as actin. Actin microfilaments , which can contract and relax , and they therefore function as tiny muscles inside the cellcells.

Flagella

A flagellum , when present, is structurally complexand contains , containing more than 250 types of proteins. Each flagellum consists of an axoneme, or cylinder, with nine outer pairs of microtubules surrounding two central microtubules. The whole cylinder axoneme is surrounded by a membrane, sometimes beset by hairs or scales. The outer pairs of microtubules are connected to the axoneme by a protein called nexin. Each of the nine outer pairs of microtubules has an a tubule and a b tubule. The a tubule has numerous molecules of the a protein called dynein that are attached along its length. Extensions of dynein, called dynein arms, connect neighbouring tubules, forming dynein cross-bridges. Dynein is involved in converting the chemical energy of adenosine triphosphate (ATP) into the mechanical energy that permits mediates flagellar movement. The scales and hairs apparently aid in swimming. The swellings and para-axonemal structures (crystalline rods and noncrystalline rods and sheets) are often involved in photoreception, providing the swimming cell with a means for detecting light, toward or from which it may swim. The flagellum bends as the In the presence of ATP, dynein molecules are activated, and the flagellum bends as dynein arms on one side of the axoneme a dynein cross-bridge become activated and move up the microtubules during microtubule. This creates the power stroke. These dynein molecules are then inactivated, and those The dynein arms on the opposite side of the dynein cross-bridge are then activated and slide up , causing the the opposite microtubule. This causes the flagellum to bend in the opposite direction during the recovery stroke. The result is the Although scientists are working to discover the additional mechanisms that are involved in producing the whiplike movement characteristic of many eukaryotic flagella, the importance of dynein activation in this process has been established.

The flagellum membrane is also complex. It contains may contain special receptors called chemoreceptors that aid respond to chemical stimuli and allow the algal cell in recognizing cues ranging from environmental changes to mating partners. Often, scales, hairs, swellings, and para-axonemal structures cover the flagellum surface. The scales and hairs apparently to recognize a multitude of signals, ranging from signals carrying information about changes in the alga’s environment to signals carrying information about mating partners. On some flagella, superficial scales and hairs may aid in swimming. The Certain swellings and para-axonemal structures(, such as crystalline rods and noncrystalline rods and sheets) are often , may be involved in photoreception, providing the swimming cell with a means for detecting light, toward or from which it may swim. The flagellum membrane flows merges into the plasma ( cell ) membrane, where the nine pairs of axonemal microtubules enter the main body of the cell. Each pair At this junction, each pair of microtubules is joined by an additional microtubule, forming nine triplets. The This cylinder of nine triplets, called constituting the basal body, anchors the flagellum . Musclelike in the cell membrane. The anchorage provided by the basal body is strengthened by musclelike fibres and special microtubules ( called microtubular roots) extend from the basal body and provide a greater anchorage base. Most flagellate cells have two flagella, and therefore two basal bodies. Typically, each basal body gives rise to two sets of with microtubular roots. The orientation of the flagella and the arrangement of the musclelike fibres and microtubular roots and musclelike fibres are are important taxonomic features that can be used to classify algae and are especially important in the classification of the Chlorophyta.

Mitosis

Mitosis, or the process of replication and division of the nucleus that results in the production of genetically identical daughter cells, is relatively similar among plants and animals, but the algae have a wide diversity of mitotic features that not only set the algae apart from plants and animals but also set certain algae apart from other algae. The nuclear envelope breaks apart in some algal groups but remains intact in others. The spindle microtubules remain outside the nucleus in some algae, they enter the nucleus through holes in the nuclear envelope in other algae, and they form inside the nucleus and nuclear envelope in still other algae. The diversity and complexity of algal mitosis has been studied in detail, and it provides provide clues to a better understanding of how mitosis operates in higher plants and animals.

Cellular respiration

Cellular respiration in algae, as in all organisms, is the process by which food molecules are “burned” metabolized to obtain chemical energy for the cell. Algae, like all organisms, sustain life by using the engery from the chemical bonds of food molecules. Most algae are aerobic (i.e., they live in the presence of oxygen), although certain a few Euglenophyceae can live anaerobically in environments without oxygen. The biochemical pathways for respiration in algae are similar to those of other eukaroyteseukaryotes; the initial breakdown of food molecules(, such as sugars, fatty acids, and proteins) , occurs in the cytoplasm, but the final high-energy-releasing steps occur inside the mitochondria.

Photosynthesis and light-absorbing pigments

Photosynthesis is the process by which light energy is converted to chemical energy as whereby carbon dioxide and water are converted into organic molecules. The process occurs in almost all algae, and in fact much of the knowledge of what is known about photosynthesis was first discovered by studying the green alga Chlorella.

Photosynthesis is divided into the comprises both light reactions and the dark reactions (or Calvin -Benson cycle). During the dark reactions, carbon dioxide is bound to ribulose bisphosphate, a 5-carbon sugar with two attached phosphate groups, by the enzyme ribulose bisphosphate carboxylase. This is the initial step of a complex process leading to the formation of sugars. During the light reactions, light energy is converted into the chemical energy that is used in needed for the dark reactions.

The light reactions of many algae differ from those of land plants because these algae some of them use different pigments to harvest light. Chlorophylls absorb primarily blue and red light, whereas carotenoids absorb primarily blue and green light, and phycobiliproteins absorb primarily blue or red light. Since the amount of light absorbed depends upon the pigment composition and concentration found in the alga, some algae absorb more light at a given wavelength, and therefore, potentially, those algae can convert more light energy of that wavelength to chemical energy via photosynthesis. All algae use chlorophyll a to collect photosynthetically active light. Green algae and euglenophytes also use chlorophyll b. In addition to chlorophyll a, the remaining algae also use various combinations of other chlorophylls, chlorophyllides, carotenoids, and phycobiliproteins to collect additional light from wavelengths of the spectrum not absorbed by chlorophyll a or b. The chromophyte algae, dinoflagellates, cryptomonads (class Cryptophyceae), and the class Micromonadophyceae, for example, also use chlorophyllides. (Chlorophyllides, often incorrectly called chlorophylls, differ from true chlorophylls in lacking a that they lack the long, fat-soluble phytol tail that is characteristic of chlorophylls.) Some green algae use carotenoids for harvesting photosynthetically active light, but the Dinophyceae and chromophyte algae almost always use carotenoids. Phycobiliproteins, which appear either blue (phycocyanins) or red (phycoerythrins), are found in red algae and cryptomonads.

The effects of water on light absorption

Red wavelengths are absorbed in the first few metres of water. Blue wavelengths are more readily absorbed if the water contains average or abundant amounts of organic material. Thus, green wavelengths are often the most common light in deep water.

Chlorophylls absorb red and blue wavelengths much more strongly than they absorb green wavelengths, which is why chlorophyll-bearing plants appear green. The carotenoids and phycobiliproteins, on the other hand, strongly absorb green wavelengths. Algae with large amounts of carotenoid appear yellow to brown, those with large amounts of phycocyanin appear blue, and those with large amounts of phycoerythrin appear red.

At one time it was believed that algae with specialized green-absorbing accessory pigments outcompeted green algae and plants in deeper water. Some green algae, however, grow as well as other algae in deep water, and the deepest attached algae include green algae. The explanation of this paradox is that the cell structure of the deep-water deepwater green algae is designed to capture virtually all light, green or otherwise. Thus, while green-absorbing pigments are advantageous in deeper waters, evolutionary changes in cell structure can evidently compensate for the absence of these pigments.

Not all algae have chloroplasts and photosynthesize. The “colourless” algae obtain their energy from organic molecules, which they absorb from the environment or digest from engulfed particles. They are classified as algae, rather than fungi or protozoa, because in all other features they resemble algae.

As in Nutrient storage

As in land plants, the major carbohydrate storage product of the green algae is usually starch in the form of amylose starch or amylopectin. ( These starches are polysaccharides in which the monomer, or fundamental unit, is the sugar glucose. ) In green algae, starch consists of Green algal starch comprises more than 1,000 sugar moleculesand is stored as a solid grain inside the chloroplast. The individual sugar molecules are bound together, primarily or entirely, with an alpha linkage , joined by alpha linkages between the number 1 and number 4 carbonscarbon atoms. The cell walls of many, but not all, algae contain cellulose. ( Cellulose is formed from the same similar glucose molecules but with a beta linkage linkages between the number 1 and 4 carbons.Although all animals can digest starch, no animals can by themselves digest cellulose.

The beta linkage in cellulose causes alternating sugar molecules to be upside down, and this “flip-flop” configuration requires a special enzyme to link or unlink the sugar molecules. The enzyme is absent in animals, although cows, beavers, and other cellulose-eaters harbour intestinal bacteria that have the cellulose-digesting enzyme.)The Cryptophyceae also store amylose and amylopectinstarch. It is These starches are stored outside the chloroplast but within the surrounding membranes of the chloroplast endoplasmic recticulum. The Most Dinophyceae make starch that stains blue-black with iodine (the standard test for amylose starch), but the chemical nature has not been studied in detail. Dinophycean starch is stored store starch outside the chloroplast, often as a cap over a bulging pyrenoid. The major carbohydrate storage product of red algae is a type of starch molecule (Floridean starch) that is more highly branched than amylopectin. Floridean starch is stored as grains outside the chloroplast.

The major carbohydrate storage product of the chromophyte algae and Euglenophyceae is formed from glucose molecules interconnected with beta linkages between the number 1 and 3 carbons. These polysaccharide compounds are distinctly different from starch (and cellulose), and they are always stored outside the chloroplast. The number of glucose units in each storage product varies among the algal classes. Each , and each type is given a special name—iname—i.e., chrysolaminarin in diatoms, laminarin in brown algae, leucosin in chrysophytes, and paramylon in euglenophytes. The exact chemical constituency of the major polysaccharide storage products is unknown for the classes Bicosoecophyceae, Dictyochophyceae, Eustigmatophyceae, Raphidophyceae, Synurophyceae, and Xanthophyceae. In the chromophyte algae, the molecule is molecules are usually small (16–40 units of sugar) and is are stored in a vacuole; the paramylon of euglenophytes is larger solution in vacuoles, whereas in the euglenophyte algae, the molecules of paramylon are large (approximately 150 units of sugar) and is are stored as a grain.The algae also produce many grains.

Alternative methods of nutrient absorption

Not all algae have chloroplasts and photosynthesize. “Colourless” algae can obtain energy and food by oxidizing organic molecules, which they absorb from the environment or digest from engulfed particles. They are classified as algae, rather than fungi or protozoa, because in most other features they resemble photosynthetic algae. Algae that rely on ingestion and oxidation of organic molecules are referred to as heterotrophic algae because they depend on the organic materials produced by other organisms.

Algae also produce many other kinds of sugars and sugar alcohols, such as rhamnose, trehalose, and xylose. While they are not polysaccharides in that the monomer is not glucose, various algae store energy using these small carbohydrate , and some algae can generate energy by oxidizing these molecules.

Reproduction and life histories

Algae are formed regenerate by either sexual reproduction, involving male and female gametes or asexual reproduction; many algae can reproduce in (sex cells), by asexual reproduction, or by both ways.

Asexual reproduction is the production of progeny without the union of cells or nuclear material. Algae, especially Many small algae , can reproduce asexually by ordinary cell division or by fragmentation, while others, especially large algae, reproduce via whereas larger algae reproduce by spores. Some red algae produce monospores (walled, nonflagellate, spherical cells) that are carried by water currents and upon germination produce a new organism. The same type of spore is called an aplanospore in the Some green algae produce nonmotile spores called aplanospores. In contrast, zoospores lack true cell walls and bear one or more flagella. Zoospores are motile and can These flagella allow zoospores to swim to a favourable environment, whereas monospores and aplanospores have to rely on passive transport by water currentsfor transport.

Sexual reproduction occurs is characterized by the union of cells, nuclei, and chromosomes and genes through the process of meiosis. It can complement or replace asexual reproduction, depending upon the organism, in which progeny cells receive half of their genetic information from each parent cell. Sexual reproduction is usually regulated by environmental events. For exampleIn many species, when temperature, salinity, inorganic nutrients (e.g., phosphorus, nitrogen, and magnesium), or day length become unfavourable, sexual reproduction is induced. A sexually reproducing organism typically has two phases in its life cycle. One stage In the first stage, each cell has a single set of chromosomes (or half that of the parent) and is called haploid, whereas in the second stage each cell has two sets of chromosomes (or the same as that of the parent) and is called diploid. When one haploid gamete (sex cell) fuses with another haploid gamete during fertilization, the resulting combinationhas , with two sets of chromosomesand , is called a zygote. Either immediately or at some later time, a diploid cell directly or indirectly undergoes a special reductive cell-division process called (meiosis). Diploid cells in this stage are called sporophytes because they produce spores. During meiosis the chromosome number of a diploid sporophyte is halved, and the resulting daughter cells are haploid. At some time, immediately or later, haploid cells act directly as gametes. In algae, as in plants, a haploid vegetative cells in this stage is are called the gametophyte stage because it is the gamete-producing stage. Similarly, a diploid vegetative stage is called a sporophyte stage because it is the spore-producing stage (via meiosis).gametophytes because they produce gametes.

The life cycles of sexually reproducing algae vary; in some, the dominant stage is the sporophyte, in others it is the gametophyte. For example, Sargassum (class Phaeophyceae) has a diploid (sporophyte) body, and the haploid phase is represented by gametes. Ectocarpus (class Phaeophyceae) has alternating diploid and haploid vegetative stages, while whereas Spirogyra (class Charophyceae) has a haploid vegetative stage, and the zygote is the only diploid cell.

In freshwater organisms especially, the fertilized egg, or zygote, often passes into a dormant state called a zygospore. The zygospore has Zygospores generally have a large store of food reserves and a thick, resistant cell wall. Following the an appropriate environmental stimulus(often changes , such as a change in light, temperature, or nutrients), the zygospore germinates and starts zygospores are induced to germinate and start another period of growth.

Most algae can live for days, weeks, or months. Often, small Small algae are sometimes found in abundance during a short period of the year ; for the remainder and remain dormant during the rest of the year, some remain dormant in resistant cysts and others . In some species, the dormant form is a resistant cyst, whereas other species remain in the vegetative state but at very low population numbers. Some large, attached species are true perennials. They may lose the main body at the end of the growing season, but the attachment sitepart, or the holdfast, then produces new growth only at the beginning of the next growing season.

The red algae, as exemplified by Polysiphonia, have some of the most complex life cycles known for living organisms. Following meiosis, a haploid tetraspore, one of four spores produced following meiosis, is produced. The tetraspore germinates four haploid tetraspores are produced, which germinate to produce either a male or a female gametophyte. When mature, the male gametophyte produces special spermatangial branches that bear structures, called spermatangia, which contain spermatia, the male gametes. The female gametophyte produces special carpogonial branches that bear carpogonia, the female gametes. Fertilization occurs when the a male spermatium, carried by water currents, “bumps into” the extended portion of the a female carpogonium and the two gametes fuse. The fertilized carpogonium (the zygote) and the female gametophyte tissue around the carpogonium it develop into a basketlike or pustulelike structure , the called a carposporophyte. The carposporophyte eventually produces and releases diploid carpospores that develop into tetrasporophytes. Certain cells of the tetrasporophyte undergo meiosis to produce tetraspores, and the cycle continuesis repeated. In the life cycle of Polysiphonia, and many other red algae, there are separate male and female gametophytes, a carposporophyte growth carposporophytes that develop on the female gametophytegametophytes, and a separate tetrasporophytetetrasporophytes.

The life cycle cycles of diatomsis unique as well, which are diploid, are also unique. Diatom walls, or frustules, are composed of two overlapping parts (the valves). During cell division, two new valves form in the middle of the cell and partition the protoplasm into two parts. Consequently, the new valves are slightly generally somewhat smaller than the originals. Over , so after many successive generations, most of the cells in the growing population become are smaller than their parents. When the such diatoms reach a critically small size, sexual reproduction is may be stimulated. The small diploid cells undergo meiosis, and among pennate (thin, elliptical) diatoms the resulting haploid gametes form and fuse into a zygote, which grows quite large and forms a special kind of cell , the called an auxospore. The auxospore divides, forming two large, vegetative cells. In , and in this manner , the larger size is renewed. In centric diatoms there is marked differentiation between nonmotile female gametes, which act as egg cells, and motile (typically uniflagellate) male gametes.

Evolution and paleontology The evolutionary relationships of algae

are not well understood. Modern ultrastructural and molecular studies have added so much new and provided important information that has led to a reassessment of the evolution of algaeis being reassessed. The poor . In addition, the fossil record for some groups of algae also hinders has hindered evolutionary studies. Finally, and the realization that some algae are more closely related to protozoa or fungi than they are to other algae came late, producing confusion in evolutionary thought and delays in understanding the evolution of the algae.

The Euglenophyceae are believed to be on an ancient lineage of algae that includes some zooflagellate protozoa, which is supported by ultrastructural and molecular data. Most Some scientists consider the colourless euglenophytes to be an older group and believe that the chloroplasts were added incorporated by symbiogenesis more recently. The order of algae with the best fossil record are the Dasycladales, which are calcified unicellar forms of elegant construction dating back at least to the Triassic Period.

Some scientists consider the red algae, which bear little resemblance to any other group of organisms, to be very primitive eukaryotes that evolved from the prokaryotic blue-green algae. Evidence in support of this view are includes the nearly identical photosynthetic pigments and the very similar starches among the red algae and the blue-green algae. Many scientists, however, attribute the similarity to an endosymbiotic origin of the red algal chloroplast from a blue-green algal symbiont. Other scientists suggest that the red algae evolved from the Cryptophyceae, with the loss of flagella. It is difficult to imagine, however, the evolutionary selection of a nonflagellate stage for an aquatic organism. Still other scientists suggest that the red algae evolved from the , or from fungi by obtaining a chloroplast. Evidence in In support of this view are similarities in mitosis and in cell wall plugs, special structures inserted into holes in a the cell wall hole walls that interconnects two interconnect cells. Some evidence suggests that such plugs regulate the plug regulates movement between the two cellsintercellular movement of solutes. Ribosomal gene sequence data from studies in molecular biology suggest that the red algae arose suddenly along with the animal, fungal, and green plant (as green algae) lineages.Whatever the origin of the red algae, they bear little resemblance to any other living group.

The green algal classes are evolutionarily related, but their origin is origins are unclear. Most consider the classes class Micromonadophyceae to be the most ancient group, and some fossil data support this view. The class Ulvophyceae is also ancient, whereas the classes Charophyceae and Chlorophyceae are more recent.

The class Dinophyceae is also of uncertain origin. During the 1960s and ’70s the unusual structure and chemical composition of the nuclear DNA was of the Dinophyceae were interpreted as a very somewhat primitive featurefeatures. Some scientists even considered the Dinophyceae to be mesokaryotes (an intermediate between the prokaryotes and the eukaryotes). That ; however, this view is no longer acceptedby most scientists, and the . Their peculiar structure is considered simply an as a result of evolutionary divergence. Some scientists consider the Dinophyceae to , perhaps about 300 or 400 million years ago. The Dinophyceae may be distantly related to the chromophyte algae. Ribosomal , but ribosomal gene sequence data suggest that their closest living relatives are the ciliates, a large, complex group of ciliated protozoa. As in the case of the other algae, the It is likely that the Dinophyceae arose from nonphotosynthetic ancestors and that later some species of Dinophyceae adopted chloroplasts by symbiogenesis and thereby became capable of photosynthesis, although many of these organisms still retain the ability to ingest solid food, similar to protozoa.

The origin of the chromophyte algae is also remains unknown. Ultrastructural and molecular data suggest that they are on in a protistan lineage which diverged a long time ago. That lineage, however, apparently remained one of protozoa and later aquatic fungi until that diverged from the protozoa and aquatic fungi about 300 to 400 million years ago. At that timea chloroplast was added (, chloroplasts were incorporated, originally as a symbiont)endosymbionts, and since then the many chromophyte groups have been evolving. Fossil, ultrastructural, and ribosomal gene sequence data support this hypothesis.

The Cryptophyceae are truly an evolutionary enigma. They have no fossil record, and other phylogenetic data are conflicting. Although some workers align them near the red algae, because both groups possess phycobiliproteins in their chloroplasts, most scientists suggest that independent symbiotic origins for the red or blue colour of their chloroplasts could explain the similarity. Cryptophytes have flagellar hairs and other flagellar features that resemble those of the chromophyte algae; however, the mitochondrial structure and other ultrastructural features are distinct and argue against such a relationship.Much like the platypus, the cryptophytes appear as though they were constructed by an administrative committee.

The fossil record for the algae is not nearly as complete as it is for land plants and animals. Red algal fossils are the oldest known algal fossils. Microscopic spherical algae (Eosphaera and Huroniospora) resembling that resemble the living genus Porphyridium are known from the Gunflint Iron Formation of North America (formed about 1.9 billion years ago). Fossils that resemble modern tetraspores are known from the Amelia Dolomites of Australia (formed some 1.5 billion years ago). The best characterized fossils are the coralline red algae that are represented in fossil beds since the Precambrian eratime.

Some of the green algal classes are also very old. Organic cysts resembling modern Micromonadophyceae cysts date from about 1.2 billion years ago. Tasmanites formed the Permian “white coal,” or tasmanite, deposits of Tasmania and accumulated to a depth of several feet in deposits that extend for miles. Similar deposits in Alaska produce yield up to 150 gallons of oil per ton of sediment. The Certain Ulvophyceae fossils that date from about 1 one billion years ago and are abundant in Paleozoic rocks. Some green algae deposit calcium carbonate along on their cell walls, and these algae produced some extensive limestone formations. The Charophyceae, as represented by the large stoneworts (order Charales), date from about 400 million years ago. The oospore, the fertilized female egg, has spirals on its surface , a result of pressing against that were imprinted by the spiraling protective cells that surround surrounded the oospore. Oospores from before about 225 million years ago had right-handed spirals, while whereas those formed since that time have had left-handed spirals. The reason for the switch remains a mystery.

Fossil Dinophyceae date from the Silurian period Period (430 million years ago). Some workers consider at least a portion of the acritarchs, a group of cystlike fossils of unknown affinity, to be Dinophyceae, but most few scientists do not agree with that view. The acritarchs occurred as early as 700 million years ago.

The Chromophyta have the shortest fossil history of among the major algal groups. Some scientists believe that the group is ancient, but there are no fossils. Others whereas others point out that there is a lack of data to support this view and suggest that the group evolved recently, as indicated by fossil and molecular data. The oldest chromophyte fossils, a putative brown algaalgae, are approximately 400 million years old. Coccolithophores, coccolith-bearing members of the Prymnesiophyceae, date from the late Late Triassic epoch Epoch (230 to 208 200 million years ago), with one reported from approximately 280 million years ago. Coccolithophores were extremely abundant during the Mesozoic eraEra, contributing to deep deposits such as the White Cliffs of Doverthose that constitute the white cliffs of southeast England. Most species became extinct at the end of the Cretaceous period Period (6665.4 5 million years ago), along with the dinosaurs, and indeed there are more extinct species of coccolithophores than there are living species. The Chrysophyceae, Bacillariophyceae, and Dictyochophyceae date from about 100 million years ago, and following despite the mass extinctions 6665.4 5 million years ago, these algae flourished. Their siliceous remains form many species still flourish. In Lompoc, Calif., U.S., their siliceous remains have formed deposits of diatomite almost 0.5 kilometre km (0.3 mile) in depth. The enormous deposits and the siliceous nature of the fossils strongly suggest that these organisms evolved very recently. In fact, mammals and birds evolved before these algae, while at Mývatn in Iceland the lake bottom bears significant amounts of diatomite in the form of diatomaceous ooze, many metres in depth.

The Xanthophyceae may be even more recent, with fossils dating from about 20 million years ago.The , while fossil records of the remaining groups of algae, especially notably the Euglenophyceae and the CryptophyceaeCryptophyuceae, lack a good fossil record. These groups are without hard parts, which may explain the lack of fossils.

Classification

which lack mineralized walls, are negligible.

Classification of algae

Diagnostic features

The classification of algae into taxonomic groups is based upon the same rules as that are used for the classification of land plants. The , but the organization of groups of algae above the order level has changed substantially since 1960. Research using electron microscopes has demonstrated new and important features of differences in features, such as the flagellar apparatus, cell division process, and organelle structure and function, that are important in the classification of algae. Similarities and differences among algal, fungal, and protozoan groups have led scientists to propose major taxonomic changes, and these changes are continuing. Molecular studies, especially comparative gene sequencingstudies, have supported some of the changes that followed electron microscopic studies, but they have suggested additional change changes as well. Since 1960 the number of classes has nearly doubledand algal classification has changed constantly. Furthermore, the apparent evolutionary scattering scatter of algal groups some algae among protozoan and fungal groups implies that a natural classification of algae by themselves is impossibleas a class is impracticable.

Kingdoms are the most encompassing of the taxonomic groups, and scientists are actively debating which organisms belong in which kingdoms. Some scientists have suggested as many as 30 or more kingdoms, while others argue have argued that all eukaryotes should be combined into one large kingdom. Using cladistic analysis (a method for determining evolutionary relationships), the green algae should be grouped with the land plants, and the chromophyte algae should be grouped with the aquatic fungi and certain protozoa. The , and the Euglenophyceae are most closely related to the trypanosome flagellates, including the protozoa that cause sleeping sickness. It However, it is unclear where the red algae or cryptomonads belong. In summary, and the overall conclusion is that the algae are not all closely related, and they do not form a single evolutionary lineage devoid of other organisms.

Division-level classification, like as with kingdom-level classification, is tenuous for algae. For example, some phycologists place the classes Bacillariophyceae, Phaeophyceae, and Xanthophyceae in the division Chromophyta, while whereas others place each class in separate divisions: Bacillariophyta, Phaeophyta, and Xanthophyta. Yet, almost all phycologists agree on the definition of the respective classes Bacillariophyceae, Phaeophyceae, and Xanthophyceae. In another example, the number of classes of green algae (Chlorophyta), and the algae placed in those classes, has varied greatly since 1960. The five classes of green algae given below are accepted by a large number of phycologists, but at least an equal number of phycologists would suggest one of many alternative classification schemes. The classes are distinguished by the structure of flagellate cells (e.g., scales, angle of flagellar insertion, microtubular roots, and striated roots)of flagellate cells, the nuclear division process (mitosis), the cytoplasmic division process (cytokinesis), and the cell covering. Many scientists combine the Micromonadophyceae with the Pleurastrophyceaeand name , naming the combined group the Prasinophyceae.

Because classes are better defined and more accepted than divisions, taxonomic discussions of algae are usually conducted constrained at the class level. The divisions provided beloware , though commonly used, but they are by no means accepted by all phycologists.

“Phylum” and “division” represent the same level of organization; the former is the zoological term, the latter is the botanical term. The classification of protists continues to be debated, and a standard outline of the kingdom Protista has not been established. The differences between the classification presented below and that the classification presented in the article protist on protists (seeprotist: Annotated classificationClassification) reflect the taxonomic variations that arise from individual interpretations.

Annotated classificationDivision Chlorophyta (green algae)Chlorophylls a and b; starch stored inside chloroplast; mitochondria with flattened cristae; flagella, when present, lack tubular hairs (mastigonemes); unmineralized scales on cells or flagella of flagellates and zoospores; conservatively, between 9,000 and 12,000 species.Class ChlorophyceaePrimarily freshwater; includes Chlamydomonas, Chlorella, and Oedogonium.Class CharophyceaeIncludes the macroscopic pondweed Chara, filamentous Spirogyra, and desmids.Class MicromonadophyceaePrimarily marine; includes the smallest eukaryotic alga, Micromonas.Class PleurastrophyceaeFreshwater and marine; includes marine flagellate Tetraselmis.Class UlvophyceaePrimarily marine; includes sea lettuce Ulva.Division ChromophytaMost with chlorophyll a; one or two with chlorophyllide c; carotenoids present; storage product βbeta-1,3-linked polysaccharide outside chloroplast; mitochondria with tubular cristae; biflagellate cells and zoospores usually with tubular hairs on one flagellum; mucous organelles common.Class Bacillariophyceae (diatoms)Silica cell walls, or frustules; centric diatoms commonly planktonic and valves radially symmetrical; pennate diatomsfound attached to substrate and , usually attached or gliding over solid substrates, with valves bilaterally symmetrical; primarily in freshwater, marine, and soil environments; at least 12,000 to 15,000 living species; tens of thousands more species described from fossil diatomite deposits; Cyclotella and Thalassiosira (centrics) and Navicula and Nitzschia (pennates).Class BicosoecophyceaeMay be included in the Chrysophyceae or in the protozoan group Zoomastigophora; colourless flagellates found flagellate cells in vase-shaped loricas (wall-like coverings); cell attached to lorica using flagellum as a stalk; lorica attaches to plants, algae, animals, or water surface; in freshwater and marine; fewer than 50 species described; Bicosoeca.Class Chrysophyceae (golden algae)Many unicellular or colonial flagellates; also capsoid, coccoid, amoeboid, filamentous, parenchymatous, or plasmodial; many produce silica cysts (statospores); predominantly freshwater; approximately 1,200 species; Chrysamoeba, Chrysocapsa, and Ochromonas.Class DictyochophyceaePredominantly marine flagellates, including silicoflagellates , which are that form skeletons common in diatomite deposits; fewer than 25 described species.Order PedinellalesWhen pigmented, has 6 chloroplasts in a radial arrangement; flagella bases attach attached almost directly to nucleus.Order Dictyochales (silicoflagellates)Typically forms siliceous skeleton that appears as spiny basket in which cell sitswith siliceous skeletons like spiny baskets enclosing the cells; flagella bases attach almost directly to nucleus; silicoflagellate skeletons common in diatomite deposits; Dictyocha, Pedinella, and Pseudopedinella. Class EustigmatophyceaeNewly described, with probably more to be discovered; mostly small, pale green, and spherical; fewer than 15 species; Eustigmatos and Nannochloropsis.Class Phaeophyceae (brown algae or brown seaweeds)Microscopic Range from microscopic forms to large kelp kelps more than 60 20 metres long; more than at least 1,500 species, almost entirely all marine; Ectocarpus, Macrocystis, and Sargassum.Class Prymnesiophyceae (Haptophyceae)Many with haptonema, a hairlike appendage between two flagella; no tubular hairs; many with organic scales; some deposit calcium carbonate on scales to form coccoliths; coccolithophorids may play a role in global warming because they can remove large amounts of carbon from the ocean water; predominantly marine and planktonicspecies; approximately 300 species; more fossil coccolithophores known; Chrysochromulina, Emiliania, and Prymnesium.Class Raphidophyceae (Chloromonadophyceae)Flagellates ; with mucocysts (mucilage-releasing bodies) commonly occasionally found in freshwater forms; sharply divided between freshwater and or marine environments; fewer than 50 species; Heterosigma, Vacuolaria, and Olisthodiscus.Class SynurophyceaePreviously placed in Chrysophyceae; silica-scaled; unicellular or colonial flagellates sometimes alternating with capsoid benthic stage; cells covered with elaborately structured silica scales; approximately 250 species, with approximately 10 new described each year since 1970; Mallomonas, Synura, and Tesselaria.Class Xanthophyceae (yellow-green algae)Primarily coccoid, capsoid, or filamentous; mostly in freshwater environments; about 600 species; Bumilleriopsis, Tribonema, and Vaucheria.Division CryptophytaUnicellular flagellates.Class CryptophyceaeChlorophyll a, chlorophyllide c2, and phycobiliproteins; starch stored outside of chloroplast; mitochondria with flattened cristae; tubular hairs on 1 one or both flagella; special ejectosomes lie in a furrow or gullet near the base of flagella; cell covered with periplast, often elaborately decorated sheet or scale covering; nucleomorph may represent reduced nucleus of symbiotic organism; approximately 200 described species; Chilomonas, Cryptomonas, Falcomonas, and Rhinomonas.Division Pyrrhophyta Pyrrophyta (Dinoflagellata)Predominantly unicellular flagellates; approximately half of the species are heterotrophic rather than photosynthetic; photosynthetic forms with chlorophyll a, 1 one or more chlorophyllide c types, and peridinin or fucoxanthin; mitochondria with tubular cristae and flagella without tubular hairs; ejectile trichocysts below surface in many members; many with cellulosic plates that form an a so-called armour around cell; some bioluminescent, some containing symbionts; resting (interphase) nucleus contains permanently condensed chromosomes; several produce toxins that either kill fish or accumulate in shellfish and cause sickness or death in humans when ingested; more than 1,200 species described, most in the class Dinophyceae; Alexandrium, Dinophysis, Gonyaulax, Peridinium, and Polykrikos.Division EuglenophytaPrimarily unicellular flagellates; both photosynthetic and heterotrophic.Class EuglenophyceaeChlorophylls a and b; paramylon stored outside chloroplasts; mitochondria with paddle-shaped cristae; flagella lack tubular hairs, but some with hairlike scales; unusual pellicle covering of sliding sheets allows organisms cells to change shapeeasily; approximately 1,000 described species; Colacium, Euglena, and Eutreptiella.Division Rhodophyta (red algae or red seaweeds)Predominantly filamentous; mostly photosyntheticbut almost one-third , a few parasitic; photosynthetic species with chlorophyll a; chlorophyll d present in some species; phycobiliproteins (phycocyanin and phycoerythrin) organized into in discrete structures (phycobilisomes); starch occurs stored outside chloroplast; mitochondria with flattened cristae; flagella completely absent; coralline red algae contribute to coral reefs and coral sands; predominantly marine; approximately 4,100 described species; Bangia, Palmaria, Polysiphonia, Porphyra, and Porphyra.